Real-Time Process Control by Machine Vision

By: Christoph Franz

Fraunhofer Institute for Laser Technology ILT, Aachen, Germany

In recent years the use of modern solid-state lasers has brought about a distinct increase in operational speed in laser materials processing. Whether with scanners or fixed optics, high speeds – as far as possible in various axes at the same time – have almost become the norm. But although the movement of the optic is precisely calculated, the position of the processing point can deviate from the planned contour. Help is at hand, thanks to a process monitoring system which precisely tracks the relative movement of workpiece and optic. It enables acceleration-related deviations from the set contour and speed to be measured exactly and the numerical control system to be adjusted accordingly.

Research scientists at the Fraunhofer ILT in Aachen have developed a camera-based system which analyzes the movements of the workpiece through the optical axis of the laser beam before or during processing. It does not matter whether a fixed or scanner optic is used – in both cases the system measures the movement of the processing point on the workpiece and documents deviations from the set contour during machine setup or operation.

The process monitoring system uses image sequence frequencies of up to 10 kHz. In various applications, contours have been measured with a processing speed of up to 10 m/min (fixed optic) and up to 15 m/s (scanner optic). The deviation from a reference system was less than 3 cm/min. At present the measured data are evaluated separately. Whilst the same technology does permit real-time measurement (there are no technical barriers to this), the accuracy class of this has not yet been completely specified.

The special design of the system means that it can be used in a very wide range of applications, including laser cutting and welding, soldering, drilling, ablation, microjoining, SLM and hardening. The various modes of operation are interesting both for system integrators and for end users. On the one hand, the system can track the processing point during machine setup, enabling the planned contour to be adjusted.

On the other hand, the system permits process control during actual operation. This means not only can the processing contour be adjusted, the laser output can also be controlled to ensure an even energy input at different laser spot speeds. That is a critical factor in particular when processing thin materials. As a result, existing processes can be optimized and new processes are made possible.